Many precision systems, such as those used in semiconductor manufacturing, use linear or planar motors for positioning objects such as semiconductor wafers. Conventional planar motors are disclosed in U.S. Pat. Nos. 3,851,196, 4,654,571, 5,196,745, and 5,334,892. These patents describe planar motors that have significant limitations. For example, the planar motor of the '196 patent has limited range of motion since each motor portion of the stationary magnet array can only generate force in a single direction. Thus, each coil array must always be located above the corresponding magnet array. This limits the range of movement for a given size actuator. The motor disclosed in the '745 patent similarly requires each coil array to be located above a corresponding linear magnet array. The motor of the '571 patent includes a coil design which generates only a limited amount of force due to the layout of the coils on the stage. In addition, the design does not generate force in six degrees of freedom. The '892 patent discloses a planar motor which permits a wide range of motion, but only in a single plane.
Conventional technology also relies upon cumbersome stacked arrangements to achieve six degrees of freedom of movement. These stacked arrangements have a number of drawbacks including additional power requirements, and reduced positioning accuracy. Motors which eliminate stacked arrangements and provide six degree of freedoms over a full range of movement of the wafer stage with a single planar motor require large magnet and coil arrays to provide the force required. This results in an increase in mass of the stage and system, thus reducing the natural frequency of the system and degrading performance.
There is, therefore, a need for a compact motor which provides six degrees of freedom with high speed and precision and energy efficient operation.